Surface Ozone Variability and Meteorological Drivers at Coastal and Mountain Sites in Bulgaria

Abstract

The study is based on simultaneous measurements of surface ozone concentrations and meteorological parameters over a period of about two years in mountainous and coastal regions in Bulgaria. The average O₃ at the mountain station (Rozhen) is 48 ppb, and at the coastal station (Ahtopol), 41 ppb. It is found that extremely low and high ozone concentrations are less common in the mountainous area than in the coastal area. The diurnal cycle of O₃ at Rozhen shows small amplitude, with maximum values recorded at night, while at Ahtopol the diurnal cycle is distinct, with a maximum in the late afternoon and a minimum in the morning. The seasonal variation in ozone is well defined at both locations. There is a broader spring–summer maximum at the mountain station, while at the coastal station, the maximum is limited to the summer. Nonparametric regression analysis showed that ozone levels at Rozhen are independent of wind, while sea winds bring ozone-rich air to Ahtopol. Boosted regression tree modelling explained 87% and 81% of O₃ variability at the mountain and coastal sites, respectively, reproducing expected meteorological influences while capturing site-specific effects and variable interactions.